The present invention relates to a process for separating the product gas mixture from the catalytic synthesis of methyl mercaptan from hydrogen sulfide and methanol. The product gas mixture has a temperature of 100.degree. C. to 150.degree. C. and a pressure of 6 to 12 bar. It contains not only the desired methyl mercaptan but also the water produced in the reaction, the byproducts dimethyl sulfide and dimethyl ether and small proportions of polysulfides, as well as unreacted methanol, excess hydrogen sulfide, and the gases nitrogen, carbon dioxide, carbon monoxide and hydrogen, which are inert in this reaction. Separation of the product gas mixture into its components recovers methyl mercaptan and dimethyl sulfide, removes water and the inert gases, and recycles the unconsumed methanol and hydrogen sulfide to the synthesis reactor.
Methyl mercaptan is an important industrial intermediate for the synthesis of methionine and for production of dimethyl sulfoxide and dimethyl sulfone. At present it is produced predominantly from methanol and hydrogen sulfide by reaction in the presence of an aluminum oxide catalyst. Methyl mercaptan is usually synthesized in the gas phase at temperatures from 300.degree. C. to 500.degree. C. and at pressures from 1 to 5 bar. The catalyst is usually doped with potassium tungstate as an activator to increase its activity and selectivity. The reaction of hydrogen sulfide and methanol to produce methyl mercaptan is an exothermic process which releases 28,500 kJ per kmol methanol reacted.
The whole process of methyl mercaptan production can be divided into two parts. The first part includes pretreatment of the feed gas mixture and its conversion to methyl mercaptan. The second part includes the separation of the product gas mixture to recover methyl mercaptan, recycling of unconsumed methanol and hydrogen sulfide, and disposal of wastewater and waste gases. This invention relates to improvements in the second part of the production process.
German Patent 17 68 826 describes a process for separating the product gas mixture from methyl mercaptan synthesis, in which the product gas is distilled at a pressure not greater than 11 bar and a temperature of 10.degree. C. to 140.degree. C. The gaseous phase in this distillation consists essentially of hydrogen sulfide, inert gases, dimethyl sulfide and methyl mercaptan. Methyl mercaptan and dimethyl sulfide are washed out of the gas phase with a countercurrent stream of methanol. The remaining hydrogen sulfide and the inert gases are returned to the synthesis reactor as recycle gas. The wash methanol carrying methyl mercaptan and dimethyl sulfide is redistilled along with the distillation bottoms, which are practically free of hydrogen sulfide, and is also returned to the production process.
The methyl mercaptan which can be obtained by this process still contains up to 0.015% by weight hydrogen sulfide and up to 0.15% by weight methanol as minor impurities.
Some of the inert gases enter the production process as impurities in the hydrogen sulfide make-up gas. Inert gases also form due to decomposition of methanol in the synthesis reactor. According to German Patent 17 68 826, part of the hydrogen sulfide recycle gas is discharged and burned to avoid accumulation of the inert gases in the process. Valuable hydrogen sulfide gas is lost to the process by doing that. The sulfur dioxide formed in the burning must be removed from the exhaust gas to meet current exhaust gas standards.
The water formed in the synthesis of methyl mercaptan is used, after it is separated from the product gas mixture, to break the methyl mercaptan/methanol and dimethyl sulfide/methanol azeotropes. Finally, the water must be removed continuously from the process. Here, there is a significant problem in that the methyl mercaptan, dimethyl sulfide and polysulfides must be removed as completely as possible from the wastewater to prevent odorous emissions and environmental damage.
According to French Patent 2 477 538, the product gas mixture, after leaving the reactor, passes through two condensation stages to condense most of the gas mixture. The condensate is collected at a pressure of 7 bar and a temperature of 30.degree. C. in a phase separation tank for separation into an organic phase and an aqueous phase. The uncondensed gases rich in hydrogen sulfide are washed with methanol and then removed from the process for burning. The wash liquid is likewise collected in the phase separation tank. The organic and aqueous phases from the phase separation tank are further processed separately in distillation columns.
One problem with the technology known at the current state of the technology is the fact that the product gas mixture is not separated sharply enough into the separate streams of materials. The result is, for example, that methyl mercaptan is also recycled into the synthesis reactor along with the hydrogen sulfide recycle gas, and the inert gases removed for burning still contain high proportions of hydrogen sulfide, methyl mercaptan and methanol. Furthermore, the water being removed from the process is not pure enough to be discharged without further treatment. Very generally, this inadequate separation of the material streams causes higher power costs, as greater material flows must be recycled. Burning of valuable reactants (hydrogen sulfide, methanol) and the consequential post-treatment of the exhaust gas also incur high costs.
One object of this invention, therefore, is to separate the product gas mixture from methyl mercaptan synthesis to obtain improved separation of the product gas mixture into the individual material streams.